Goldilocks Analogue – Prototyping 3

Following my initial design article, and the follow up design article, I’ve put quite a lot of thought into how I can make this Goldilocks Analogue device best achieve my stated goals. Pictured is the new 3rd Goldilocks Analogue Prototype.
I’m now working on the 4th Goldilocks Analogue Prototype. The PCB layout has been completed, and we’re talking about how to produce it with a view that this will become the final and possibly production version.
The finished 3rd Prototype boards are now in my hands, and testing of the PCB configuration the new SPI EEPROM and SRAM capabilities, together with MSPIM interface for the DAC begins. These two features contribute to making the Goldilocks Analogue great analogue synthesiser platform. Combined with a Gameduino2 LCD and touch screen, it creates flexible sound touch controller, with quality analogue stereo output.This is the working design document. It will grow as I get more stuff done, and notes added here. I’ve pretty much finished the paper design now, and will let it settle for a few weeks over the 2014 holiday season. It is sometimes good to do things again, with a few weeks perspective from the original decisions.

Major Revision in Strategy

Over the past months I’ve been spending time writing code to go along with the latest revision of the Goldilocks Analogue. I have successfully implemented a version of the NASA EEFS simple flash file system, to use to buffer data either for acquisition or for analogue playback, and I’ve been working on streaming functions to get data off the SD card and off the EEFS flash file system. The outcome is that it is not possible to do everything with just one SPI bus, and keep generality when needed. The SD card is just too slow, and can’t be easily interrupted. The FRAM/SRAM/EEPROM doesn’t have enough storage to effectively stream GigaBytes of data, as a uSD card can achieve.
So, what to do? Adafruit uses a software bit-banged SPI outcome to drive their MCP4921 and doesn’t get close to the maximum speed I want to achieve. Fortunately, with the ATmega1284p there is a simple answer at hand. I have decided to move the MCP4822 off the standard SPI pins, and connect it to the USART1 TX and XCK pins, using the USART in its Master SPI mode.
This is a major revision in strategy. Previously I have been very adverse to putting anything on the standard Arduino pins, preferring to keep all of the Goldilocks extra features off the Arduino footprint. However, the outcome is well worth using the USART1 to drive the MCP4822, and nothing is compromised.
USART MSPI mode is available on any ATmega device. On the UNO platform, using the ATmega328p, there is only one USART and so of course it is reserved for serial communications. The Goldilocks ATmega1284p has two USART interfaces, and usually the second one (USART 1) goes unused. Therefore connecting its XCK and TX pins to the MCP4822 is the simplest and best outcome to achieve high throughput and regularity SPI output on a non-shared SPI interface. And, as the MCP4822 DAC has high impedence (~10kOhm) inputs, having the DAC sharing the pins won’t affect normal pin usage to any extent.
And, there’s more win. The USART MSPI has double buffering for the transmit function. This means that we can actually achieve a higher throughput using the USART MSPI than we can using the standard SPI bus! These logic traces demonstrate that the my best implementation of the SPI interface requires 4.58us to transmit a “frame” of information, consisting of two 12 bit samples. Using the USART MSPI interface we can achieve 4.25us per frame.
For more detail: Goldilocks Analogue – Prototyping 3